In order to determine how the kidneys adjust to changing physiologic needs over a long period, it is standard to collect all urine produced for 24 hours. Since substances are excreted by the kidneys at different rates and amounts, in particular the concentration of catecholamine and its derivatives, during the day such a long-term collection of all urine generated over a 24-hour period provides a wealth of usable and normally fairly accurate information.
The catecholamine level in particular is important in detection and treatment of sympatho-adrenal tumors. Catecholamine is however fairly fragile so that it must be protected from excessive temperatures, light, or oxygen or it will break down. Thus when catecholamine levels are being checked, the urine-collection container is made opaque and is provided with a reagent, normally an acid, serving to stabilize the catecholamine. Such a stabilizer is not, however, needed when the urine is being collected to test for a kidney stone as acidification is not needed.
Thus to collect urine over a 24-hour period on an out-patient basis, the patient must be provided with a can of 2 to 3 liter capacity. Once the 24-hour sample has been collected and all of the deposits have been mixed together, the laboratory decants a portion of the contents into a smaller bottle and tests it, normally by centrifuging to start with. The balance in the large can is dumped. This decanting is particularly difficult as the bigger can has a wide mouth to ease urinating into it, so that accurately pouring into a much smaller bottle is virtually impossible to do without making a mess. Furthermore just urinating into the large container can be a problem, especially once it is largely filled and, thus, fairly heavy.
These problems are all compounded by the use of a stabilizing reagent used for acidification of the combined specimens. Typically nitric, sulfuric, or acetic acid at a fairly high concentration is used. Such an acid is normally poured or provided in the large collection can at the start of the collection period, so that the user must be careful not to spill any of this acid and to avoid any splashback. Acid burns are thus a significant risk. When the large can is first used the acid is at maximum concentration so that the acid-burn risk is at its greatest. This risk decreases as the can is filled, but the ever-heavier can becomes increasingly difficult to hold and use, particularly for a female patient.